1. Field of the Invention
The present invention relates to a thermal printer in which paper is clamped between a printing head and a platen to perform printing and in which a platen release mechanism for separating the platen from the printing head is provided.
2. Description of the Related Art
A thermal printer in which paper is clamped between a printing head and a platen to perform printing is sometimes used as a printer unit mounted on a cash register for printing onto a receipt.
In such a thermal printer, in some cases, there is provided a platen release mechanism for attaching/detaching the platen to/from the printing head, whereby the platen is separated from the printing head during replacement of paper. When new paper is laid on the printing head, the platen is returned to its original position to thereby set the paper.
FIG. 8 is a side elevational view illustrating a conventional platen release mechanism.
In the platen release mechanism, right and left bearings 82 for rotatably holding a shaft 80a of a platen 80 are held by a platen frame 84 with back and forth play, and this platen frame 84 is rotated to separate the platen 80 from a printing head 90 or fix the platen to the normal operational position where it is brought into contact with the printing head 90. Under the condition that the platen 80 is fixed in the normal operational position, the above-described bearings 82 are engaged with engagement grooves 86A formed in the body frame 86 of the thermal printer and the bearings 82 are fixed under the condition that the bearings are hooked and retained at lock arms 92.
However, in the conventional platen release mechanism, since portions engaging with the engagement grooves 86A in the bearings 82 of the platen 80 are in cylindrical shapes, in the case where frictional force is reduced due to, for example, adhesion of oily components to these portions, there is a possibility that the bearings 82 are rotated under the condition that the bearings are fitted with the engagement grooves 86A in accordance with the rotational drive of the platen 80. When the platen bearings 82 are rotated, the portions of the engagement grooves 86A and the bearings 82 are worn out to cause reduction in durability and the rotary center of the platen 80 is displaced to adversely affect the feed amount of paper to generate degradation in printing quality and printing faults.
An object of this invention is to prevent the idle rotation of platen bearings while keeping the detaching and attaching operationability of a platen in a thermal printer provided with a platen release mechanism for separating the platen away from a printing head.
In order to achieve the above-mentioned object, according to the present invention, there is provided a thermal printer for printing on paper clamped between a printing head and a platen, comprising a bearing for rotatably holding a shaft of the platen, a movable platen frame for holding the bearing, an engagement groove that is provided on a body frame and is engageable with an engagement portion of the bearing in a state in which the platen is moved to a normal operation position, and a lock arm for retaining a part of the bearing and for fixing the bearing in engagement with the engagement groove, with the thermal printer being provided with a platen release mechanism for allowing the platen to be attachable to and detachable from the normal operation position, in which the engagement portion of the bearing is formed into a point-symmetrical shape with respect to a celtral axis of a platen shaft and a non-circular shape (for example, rhombus or ellipsoid) as viewed from the axial direction of the platen shaft and the engagement groove is formed into a shape corresponding to the engagement portion of the bearing.
Also, the engagement portion of the bearing is formed into a line-symmetrical shape with respect to a straight line passing through a center of the platen shaft and a non-circular shape (for example, a regular triangular shape or a regular pentagonal shape) as viewed from axial direction of the platen shaft and the engagement groove is formed into a shape corresponding to the engagement portion of the bearing.
According to such structure, since the bearings are not rotated on the engagement grooves, it is possible to prevent the degradation in durability due to wearing of the bearings and the frame, and to further stabilize the rotation of the platen to thereby make it possible to enhance the printing quality. Furthermore, in the conventional structure, in order to prevent rotation of the bearings, it is necessary to increase the thickness of the frame or to increase the pressing force between the bearings and the frame to thereby increase the frictional resistance between the bearings and the engagement grooves as much as possible. However, according to the present invention, since such countermeasures are not necessary, it is possible to reduce the thickness of the frame for miniaturization and reduction in cost. Also, since the pressing force between the bearing and the frame may be reduced, it is possible to use material that has a relatively low rigidity for the frame or other members, to thereby make it possible to reduce the cost.
Also, the shape of the bearings is formed to be point-symmetrical with respect to the center of the platen shaft or to be line-symmetrical with respect to the straight line passing through the center, so that even if the bearings are held to the platen frame under the condition that a play is given and in a rotatable state, it is easy to engage the bearings and the engagement grooves in the normal orientation.
It is preferable that the engagement portion of the bearing is formed into a longitudinal shape as viewed from the axial direction of the platen shaft. With such a structure, when the bearing is engaged with the engagement groove, a rotational angle of the bearing is likely to be determined so that it is possible to avoid the case where the bearing is forcibly engaged with the engagement groove under the condition that the bearing and the engagement groove are not normally engaged with each other.
It is further preferable that a size of an introduction port of the engagement groove is set so as to be smaller than a longer width of the engagement portion of the bearing. With such a structure, it is possible to completely avoid the engagement in an incorrect orientation between the bearing and the engagement groove.
It is further preferable that a contact member brought into contact with one side of the engagement portion for correcting the orientation of the engagement portion of the bearing is provided on a forward side of the engagement groove on a path along which the engagement portion of the bearing is moved.
With such a means, even if the orientation of the bearing is one in which the bearing is not engaged with the engagement groove, the above-described contact member is brought into contact with the bearing when the bearing is caused to be close to the engagement groove, so that the bearing is automatically corrected to the an orientation in which the bearing is engaged with the engagement groove. Accordingly, it is possible to always engage the bearing and the engagement groove in a normal direction.